This invention relates generally to fluid processing and related measurements. Specifically, the invention concerns a reduced vibration sensor tube for measuring a process parameter in a fluid flow. The sensor tube is configured to reduce coherent vortex shedding and reduce flow-induced vibrations. This lowers structural demands on the sensor tube, increases sensor and sensor tube service life, and improves signal quality by reducing vibration-induced noise.
Safe, accurate, and cost-effective fluid measurements are important to a wide range of industrial and scientific processes. Many of these applications require measurements using sensor tubes such thermowells, Pitot tubes and similar structures, which are positioned directly in a process flow stream in order to communicate a process parameter to a process sensor, in order to monitor the process parameter.
Process parameters are physical variables such as temperatures and pressures, which typically characterize a process fluid. Process sensors are used to sense or measure the process parameters by generating sensor signals as functions of the parameters. Typical sensors include thermocouples, resistance-temperature detectors, pressure transducers, flow sensors, PH sensors and other sensor devices configured to sense or characterize a wide range of process fluid parameters and other process variables.
In some applications, a sensor tube and a sensor make up a standalone sensor module. In other applications, the sensor module is combined with a mounting structure and a transmitter/connection head with a controller and input/output (I/O) interface, in a configuration typically referred to as a field device. Field devices typically perform additional signal processing and monitoring functions, generate higher-order outputs for communication with process measurement and control systems. In some configurations, field devices also perform process control functions. Representative sensor modules and field devices are available from a number of commercial inventors, including, for example, Rosemount Inc. of Chanhassen, Minn., a division of Emerson Process Management.
Because thermowells, Pitot tubes, and other sensor tube structures are situated directly in the process flow, they are subject to a number of stress factors including flow-induced vibrations. Flow-induced vibrations typically arise as a result of vortex shedding and other turbulent wake field effects, which generate periodically alternating forces on the sensor tube. These forces cause the tube to oscillate back and forth or vibrate, increasing mechanical stress and reducing service life for both the sensor tube and its associated sensor. Flow-induced vibrations are particularly problematic when they occur near a natural resonant frequency, producing forced resonant oscillations that can result in catastrophic failure. Even relatively small oscillations can also be an issue, particularly when combined with other stresses such as high drag forces or static pressure gradients, or with corrosion, fatigue, or erosion of the sensor tube structure.
Previously, the problem of sensor tube vibrations was addressed by increasing the strength of the sensor tube. This approach requires thicker tube walls or specialized construction, which increases cost, expands the devices' size and weight envelope, decreases sensitivity and increases response time. There is thus a need for flow-induced vibration reduction techniques that are not limited to mechanical strengthening, and are applicable to a range of different sensor tube configurations.